An Autonomous System (AS) comprises a collection of network devices which share the same routing protocol to exchange routing information. A network device may for example be a router, a switch, or a computer, etc. The routing protocols uses within an AS are Interior Gateway Protocols (IGP). Mainstream IGPs include Open Shortest Path First (OSPF) protocol and Intermediate System to Intermediate System (ISIS) protocol. An AS may be divided into multiple areas. In an OSPF system, an Area Border Router (ABR) connects a backbone area of the AS to a non-backbone area, and an Autonomous System Border Router (ASBR) exchanges routing information with another AS. In an ISIS system, routers of the same level establish neighbour relationships. Further, Level-1-2 routers are capable of establishing neighbour relationships with Level-1 routers, Level-2 routers as well as Level-1-2 routers, and Level-2 routers may communicate directly with routers outside the AS.